[GRASS-dev] GRASS inefficiency and FFTW

[Glynn Clements]

stefano de paoli wrote:

> > FFTW itself is free software, and it has
> > substantially better
> > performance than the old NR FFT implementation (or
> > just about any
> > other implementation in existence, AFAICT). The
> > inefficiency is a
> > consequence of implementing a "minimal" change
> > (preserving the
> > existing fft() interface) rather than an "ideal" one
> > (modifying the
> > callers to use a more appropriate interface).
> 
> 
> Someone else in this thread suggested that the minimal
> Vs the ideal change might be due to the "development
> cost". 
> As far as I know you did the change. Was the minimal
> change really a choice of development cost or
> something else?

Developement cost ("cost" in the sense of effort; I don't get paid for
this).

Changing the API would have required changing a (small) number of
modules, none of which were familiar to me. This would have required
additional effort, and risked introducing bugs. OTOH, the additional
memory consumption was considered a relatively minor issue (in terms
of performance, copying an array is a lot cheaper than computing an
FFT on it).

IIRC, the possibility that the increased memory usage would exceed the
memory available to the application wasn't considered at the time. 
Presumably, the issue of memory consumption wasn't a common problem at
that time, otherwise we would have scrapped the NR code earlier.

-- 
Glynn Clements <glynn at gclements.plus.com>